Contemporary capacitors have a layered structure that starts with a valve metal or film, usually of aluminum or tantalum. The surface of that film is typically oxidized under controlled conditions ("anodizing") to form a high surface area dielectric layer made of the oxidized anode metal. A solid counter electrode is applied over the dielectric, and a metal electrode formed on the counter electrode.
Tantalum capacitors are often made from sintered powder compacts and suspended in an electrolyte solution, i.e., anodized, under appropriate current density to produce the anodic oxide dielectric. This anodizing step may be carried out at a temperature up to about 95.degree. C. in an electrolyte which typically consists of a dilute aqueous or mixed aqueous/ethylene glycol solution of a mineral acid or a salt of a mineral acid (e.g., phosphoric, sulfuric, nitric or hydrochloric acid) at an applied voltage that is 3-4 times the rated voltage of the part. Electrolytes which tend to give the best results (i.e., highest dielectric quality) often contain 50-60 vol % ethylene glycol or polyethylene glycol and 0.5 to 2 or more vol % phosphoric acid and are maintained at a temperature between 80.degree. and 90.degree. C. The purpose of the anodizing process is the formation of a solid dielectric layer on the surface of the anode. The dielectric is then covered with a counter electrode and then a layer of carbon and silver.
The solid counter electrode layer represents a balance between adequate conductivity (to provide electrical connection between the dielectric layer and the carbon/silver) and insulation (i.e., sealing off defects in the dielectric layer that would otherwise cause a short circuit between the anode and the electrode). Commercial usage requires that these functions be performed with a leakage current that is less than 1% of the product of the voltage and the capacitance (in microfarads), i.e., &lt;0.01 CV. Leakage currents are thought to result from pinhole surface defects that allow current to flow through the dielectric layer.
One material that is typically used as the solid counter electrode is manganese dioxide. Although somewhat low in conductivity, manganese dioxide becomes an electrical insulator when a defect causes a short and localized heating near the defect site. To offset the conductivity, manganese dioxide is often mixed with an electrically conductive organic complex. See, U.S. Pat. No. 5,567,209.
Manganese dioxide is formed as a solid electrolyte on the surface of the anodic oxide film by impregnating the anode with manganese nitrate, thermally decomposing the nitrate to the oxide, and reforming the anode. The purpose of the reformation is to lower the leakage current in a controlled manner. Typically, the reformation is performed with manganese dioxide at conditions that include an applied voltage that is less than 50-55% of the initial formation voltage.
In an effort to find a solid electrolyte with a lower resistivity, manganese dioxide has been replaced with an electrically conductive, film forming organic material (e.g., polyacetylene, poly-p-phenylene, polypyrrole, polythiophene, and polyaniline and their derivatives) either with or without a dopant material. See U.S. Pat. Nos. 5,567,209 and 5,436,796 the disclosures of which are herein incorporated by reference.
U.S. Pat. No. 5,567,209 repeatedly impregnates a tantalum capacitor with a polyaniline salt monomer solution followed by a polymerization step. A carbon or graphite paste may also be applied. Example 1 shows a formation voltage of 48 V, and example 2 identifies a formation voltage of 13 V. None of the examples uses a reformation voltage.
U.S. Pat. No. 5,436,796 describes a solid electrolytic capacitor that uses an electrically conductive composite film containing a reduced polyaniline and 10-300% of a second polymer. The second polymer is said to help reduce leakage current from the polyaniline alone (0.1-0.5 CV) to 0.001 CV or less. The polymeric film is formed on the anode surface, washed of by-products, dried at elevated temperature, and imprinted with an electrode pattern. Examples 2-4 show that applying a reformation voltage to the formed capacitive element at 70% of the formation voltage under an atmosphere of 90% relative humidity reduced the leakage current value of the capacitor.
It would be useful to have an efficient manufacturing process for capacitive elements containing a polymeric electrolyte that could be readily integrated into an existing manufacturing line without significant capital expenses.